Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/209

1869.] SEARLES WOOD BOULDER-CLAY. 109 2. Down the valley of the Lune to Sedbergh, thence up the valley of the Rawthey, or up that of the Clough, and so over into the valley of the Ure.

3. From Sedbergh up the valley of the Dee, and so over into the valley of the Widdale beck, and thence into Wensleydale.

4. Down the valley of the Lune to below Kirkby Lonsdale, thence up the valley of the Greta and that of the Dale beck, over into the valley of the Gale beck, and thence over again into the valley of the Widdale beck.

5. From the Gale beck up the Cam beck, and so over into Wharfedale.

6. From the valley of the Greta (as in No. 4) up that of the Wenning, and thence either by the valley of the Anstwick beck, or that of the Fen beck, over into the valley of the Ribble, and thence over again into the valleys of various becks that are tributary to the Aire.

In the first and second of these routes, the highest ground required to be covered by the sea seems about 1200 feet, in the third and fourth it seems about 1500 feet, in the fifth between 1200 and 1300 feet, but in the sixth there seems to be no barrier above 800 feet, while Stainmoor Pass is 1400 feet above the sea-level. The height of the top of Wasdale Craig (the original source of the boulders, according to Prof. Phillips) is marked on the Ordnance map as 1479 feet.

The lowest of these several elevations would accord with the phenomena discussed in this paper, since it would place the chalk Wold below the sea-level. I conceive, however, that a considerable depth of sea-water would be necessary to waste the ice out of the valleys along the transit route previously occupied by the ice-sheet; and until this was effected a floating transit could not take place; but it is remarkable that, according to those who have studied the distribution of the Shap boulders, they do not seem to occur in Airedale, along which dale the route requiring the least submergence passes, and that it is by the more elevated route of Stainmoor that most of these boulders have come into the east of Yorkshire. These apparently conflicting phenomena, and similar phenomena attending the non-transit of the blocks over the watershed between the Lune and the Kent, referred to by Prof. Phillips*, appear to me susceptible of a satisfactory explanation by supposing the lower routes to have continued blocked by ice after a higher route by which the blocks have passed, such as Stainmoor, had become free from it, and so remained until the source of the blocks had become submerged†. If that explanation be the true one, it would seem to follow that, during the Postglacial emergence, no ice existed in these parts adequate to the transport of the blocks until after the lowest of these routes had reached the surface otherwise we should expect their transit by the lower routes to have occurred

relative elevations of the country around Wasdale Craig must have greatly altered since the diffusion of the blocks, in order to reconcile the places of their occurrence.
 * See Report of Brit. Assoc, for 1804, p. 65. The Professor argues that the

† The depth at which ice floats would enable it to pick up blocks even after the parent source of them had become submerged; and these, by the many turnings over and squeezings up which occur to floating ice, might become transferred to ice of so much less draught of water that they could pass even higher elevations than the original source.